A spinal fusion is a surgical procedure that promotes two back bones, or vertebrae, growing together into one bone. FIG. 1 models the front or anteroposterior view of two such vertebrae L5 (104) and S1 (108) separated by a disc 112. FIG. 2 models the side or lateral view of the same. As shown in FIG. 3, in the course of the spinal fusion, a fusion cage 304 is at times required.
The fusion cage 304 is a medical implant that is used to replace the removed disc between two vertebrae or to replace one or more vertebrae and their adjacent disc. Fenestrations 308 in these cages are usually filled with a bone grafting material that promotes a bony fusion between the bone above the cage and the bone below the cage. A screwhole 312 is usually provided in the front of the cage to accommodate an insertion handle used in positioning the cage into the disc space. Fusion cages are supplied in various sizes to fit the space between the bones being fused. As shown in FIG. 4, a trial cage 404 can be used to select the desired size fusion cage to fit into the disc space.
In some situations it is desirable to mechanically fix together the bones above and below the cage in order to limit movement between the bones and cages, thus promoting successful fusions and preventing cage displacement. This fixation can be accomplished as shown in FIG. 5 by passing a guide pin 504 into the bone on the near side of the cage, through a fenestration 308 in the interior of the fusion cage 304, and then into the bone on the far side of the cage. A fixation screw 508 can then be inserted over the guide pin 504 after which the guide pin is then removed. As FIG. 6 illustrates, the cage fenestration 308 cannot be visually acquired with the cage 304 in place in the disc. The trajectory of the guide pin 504, starting from the visually acquired guide pin entry point 604, must instead be directed with the use of x-rays.
In order to visualize the location of the cage fenestration 308 (FIG. 5) through which a fixation screw 508 is to pass, metal markers are placed in the wall of the fusion cage 304 that are immediately adjacent to this fenestration 308.
FIG. 7 illustrates the front view of the spine shown in FIG. 6 as it would appear on an x-ray. The L5 backbone 104 and S1 backbone 108 would be seen. Since the fusion cage 304 and the remaining disc 112 are invisible on x-ray, an empty space would appear in their place. Because, like bone, metal is visible on x-ray, metal markers in the walls of the cage fenestration 308 would be visible. The surgeon would be able to acquire the right side wall marker 704 and the left side wall marker 708.
FIG. 8 illustrates the side view of the spine pictured in FIG. 6 as it would appear on an x-ray. The L5 backbone 104 and the S1 backbone 108 would be visible. Again, an empty space would appear in the place of fusion cage 304 and any remaining disc 112. On the x-ray, the surgeon would be able to acquire the additional front wall marker 804 and the rear wall marker 808. As depicted in FIG. 8, these markers can be made in a different shape in order to distinguish them from the side wall markers 704 and 708 shown in FIG. 7.
Being metallic, the guide pin 504 can also be seen on an x-ray. Using the front x-ray view exemplified by that shown in FIG. 7, a surgeon would direct the guide pin 504 between the right side wall marker 704 and the left side wall marker 708. Using the side x-ray view FIG. 8, the surgeon simultaneously directs the guide pin 504 between the front wall marker 804 and the rear wall marker 808.
The surgeon would thus be assured that s/he has passed the guide pin 504 through the L5 backbone 104, through the cage fenestration 308 in the fusion cage 304, and into the S1 backbone 108. The surgeon can then insert the fixation screw 508 down over the guide pin 504 as shown in FIG. 7 and FIG. 8. The guide pin 504 would then be removed, leaving the fixation screw 508 in position passing through the cage fenestration 308 in the fusion cage 304.
Unfortunately, due to the difficulty in directing a guide pin 504 through a screwhole fenestration 308 in a fusion cage 304 using x-rays, the screwhole fenestration 308 must be significantly larger than the fixation screw 508. As a result, it is possible for the fusion cage 304 to partially displace out of the disc 112. This displacement can then result in excessive movement between the L5 backbone 104 and the S1 backbone 108, resulting in a failure of the spinal fusion. As shown in FIG. 5, a large screwhole fenestration 308 leaves any remaining fenestrations in the fusion cage 304 to be small. This results in most of the bone grafting material being placed in the screwhole fenestration 308, which is unwanted.
Further, passage of the fixation screw 508 through the screwhole fenestration 308 can disturb this bone grafting material and adversely impact a successful spinal fusion. It is therefore desirable to make the screwhole fenestration 308 as small as possible in order to prevent cage migration, and to allow the remaining fenestrations to be as large as possible and to carry the majority of the bone grafting material.